The present invention is generally directed to a process for the fabrication of thin film field effect transistors employed in matrix addressed liquid crystal displays. More particularly, the present invention is directed to the utilization of specific materials in the field effect transistor (FET) fabrication process and structure. Even more particularly, the present invention is directed to the solution of material process compatibility problems and to the problem of pixel element discharge during off cycles.
A liquid crystal display device typically comprises a pair of flat panels sealed at their outer edges and containing a quantity of liquid crystal material. These liquid crystal materials typically fall into two categories: dichroic dyes and a guest/host system or twisted nematic materials. The flat panels generally possess transparent electrode material disposed on their inner surfaces in predetermined patterns. One panel is often covered completely by a single transparent "ground plane" electrode. The opposite panel is configured with an array of transparent electrodes, referred to herein as "pixel" (picture element) electrodes. Thus, a typical cell in a liquid crystal display includes liquid crystal material disposed between a pixel electrode and a ground electrode forming, in effect, a capacitor like structure disposed between transparent front and back panels. In general, however, transparency is required for only one of the two panels and the electrodes disposed thereon.
In operation, the orientation of liquid crystal material is affected by voltages applied across the electrodes on either side of the liquid crystal material. Typically, voltage applied to the pixel electrode effects a change in the optical properties of the liquid crystal material. This optical change causes the display of information on the liquid crystal display (LCD) screen. In conventional digital watch displays and in newer LCD display screens used in some miniature television receivers, the visual effect is typically produced by variations in reflected light. However, the utilization of transparent front and back panels and transparent electrodes also permits the visual effects to be produced by transmissive effects. These transmissive effects may be facilitated by separately powered light sources for the display, including fluorescent light type devices. LCD display screens may also be employed to produce color images throught the incorporation of color filter mosaics in registration with the pixel electrode array. Some of the structures may employ polarizing filters to either enhance or provide the desired visual effect.
Various electrical mechanisms are employed to sequentially turn on and off individual pixel elements in an LCD display. For example, metal oxide varistor devices have been employed for this purpose. However, the utilization of thin film semiconductor switch elements is most relevant herein. In particular, the switch element of the present invention comprises a thin film field effect transistor employing a layer of amorphous silicon. These devices are preferred in LCD devices because of their potentially small size, low power consumption, switching speeds, ease of fabrication, and compatibility with conventional LCD structures. However, fabrication processes for certain desired semiconductor switch element structures have been found to be incompatible with the employment of certain materials used in the transparent LCD electrodes. It is seen that while certain physical FET structures or LCD devices are desirable, it is often extremely difficult to devise processes that satisfactorily produce the desired structure. In particular, in any process of the kind contemplated herein, the number of masking steps is desired to be low since, in general, the greater the process complexity the lower is the reliability of the resulting device and the process yield.
One of the significant materials problems that can arise in the fabrication of thin film FETs for LCD screens is the problem of providing good electrical contact between source and drain line metal and the amorphous silicon layer of the FET. In general, molybdenum is a desirable metal to employ for source and drain electrode pads, but molybdenum may not exhibit good electrical contact with intrinsic amorphous silicon. A thin layer of aluminum disposed between the molybdenum and the amorphous silicon may be provided as discussed in concurrently filed application Ser. No. 761,939 filed Aug. 2, 1985, which is assigned to the same assignee. However, care must be taken to avoid etchant compatibility problems with indium tin oxide employed for the pixel electrodes. Moreover, aluminum has a tendency to diffuse into the silicon material, thus potentially degrading device performance, particularly if high process temperatures are employed in subsequent process steps.
Another significant problem encountered in LCD devices is the tendency for capacitive discharge to occur during off cycles. In this situation, the capacitor formed by the pixel electrode, the ground plane electrode and the liquid crystal material as a dielectric, tends to discharge through the FET if the FET device characteristics are inappropriate. In particular, it is desirable to limit FET current under conditions of reverse gate voltage. If the source drain current is high under these conditions capacitive leakage tends to occur and this can affect display quality. It is also desirable that the current voltage characteristics do not exhibit large hysteresis loops, since this can result in voltage uncertainty on the pixel electrode.
Further, there are a number of papers which discuss amorphous silicon FETs with N.sup.+ amorphous silicon for matrix addressed liquid crystal display applications. These papers include the following: "Proceedings of the 1982 International Display Research Conference" by A. Lakatos, pages 146-151, IEEE (1982); "Society for Information Display (SID) Digest" by Kouji Souzuki, pages 146, 147 (1983); "Applied Physics", Volume 24, page 357, by Snell et al. (1981); "Elec. Letter", Volume 18, No. 20, by Stroomer et al. (Sept. 1982); "Proceedings of the Third International Display Research Conference", Paper No. 5.3, by M. Sugata et al., SID and ITE (Oct. 1983). However, none of these papers describe use of the specific materials and process described herein.